1. Field of the Invention
The present invention relates to battery chargers and methods for charging a battery. The present invention relates, in particular, to battery chargers and charging methods suitable for charging a battery, such as a nickel metal hydride battery, that generates a relatively large amount of heat during charging.
2. Description of the Related Art
Rechargeable batteries have been recently used, e.g., as a power source for power tools. Nickel metal hydride batteries have been widely used as power tool batteries. To charge the battery, a battery charger for rapidly charging the battery using a relatively high current has been used. That is, while one battery is being rapidly charged for about 20 minutes, the power tool can be continuously used by replacing the depleted battery with another battery that has been recharged.
The inventor of the present invention studied ways to improve the performance of a power tool by using, as a battery, a nickel metal hydride battery. Although nickel metal hydride batteries can provide higher storage capacity than a nickel-cadmium battery, a large amount of heat is typically generated during charging. If the battery temperature becomes excessively high due to the generated heat, the electrodes and separators of cells within the battery will deteriorate and battery life will be shortened. Due to this reason, nickel metal hydride batteries cannot be rapidly charged using a high current according to known charging techniques designed for a nickel-cadmium battery.
Furthermore, nickel metal hydride batteries are more sensitive to overcharging than a nickel-cadmium battery and overcharging reduces the battery life. For that reason, it is required to avoid overcharging the nickel metal hydride battery. However, if the equipment does not require battery replacement, the charging current and the discharging current may be integrated and the battery is charged based on die integrated value in order to avoid overcharging the battery. Thus, the battery can be fully charged. On the other hand, if the equipment such as the above-stated power tool, requires replacing one battery with another, it has been difficult to fully charge the battery without overcharging it using known charging techniques.
In one aspect of the present teachings, battery chargers and battery charging methods are taught that permit rapid and complete charging of the battery without overheating the battery. In one embodiment, a battery charger is taught that includes:
a memory storing a map of allowable current values that are selected based upon the current battery temperature and the current battery temperature increase rate, with which a battery can be charged in order to prevent the battery temperature from excessively rising;
a temperature detection device detecting the current temperature of the battery;
a temperature increase rate output device calculating the battery temperature increase rate based upon the battery temperature detected by the temperature detection device;
an allowable current retrieval device retrieving the allowable current value from the map based upon the battery temperature detected by the temperature detection device and the battery temperature increase rate calculated by the temperature increase rate output device; and
a charging device charging the battery with the allowable current value retrieved by the allowable current retrieval device.
In another embodiment, a battery charger is taught that includes:
a memory storing a map of allowable current values that are selected based upon the current battery temperature and the current battery temperature increase rate, with which a battery can be charged in order to prevent the battery temperature from excessively rising, the map including relatively low allowable current values when the battery temperature is relatively high and the battery temperature increase rate is relatively large;
a temperature detection device detecting the current temperature of the battery;
a temperature increase rate output device calculating the battery temperature increase rate based upon the battery temperature detected by the temperature detection device;
an allowable current retrieval device retrieving the allowable current value from the map based upon the battery temperature detected by the temperature detection device and the battery temperature increase rate calculated by the temperature increase rate output device;
a charging device charging the battery with the allowable current value retrieved by the allowable current retrieval device;
a charging completion determination device determining that charging has been completed based upon whether or not the battery temperature detected by the temperature detection device and the battery temperature increase rate calculated by the temperature increase rate output device fall within a region indicating a final charging stage (i.e., a region in which the battery temperature increase rate is relatively large and a relatively low allowable current value has been selected) at a relatively high frequency; and
a charging concluding device stopping the battery charging when the charging completion determination device has determined that charging is completed.
In another embodiment, a charging method is taught that includes:
detecting the current temperature of a battery;
calculating the battery temperature increase rate from the detected temperature;
selecting an allowable current value based upon the detected temperature and the calculated battery temperature increase rate, with which the battery can be charged while preventing the battery temperature from excessively rising; and
charging the battery with the retrieved allowable current value.
In another embodiment, a charging method is taught that includes:
detecting the current temperature of a battery;
calculating the battery temperature increase rate from the detected temperature;
selecting an allowable current value based upon the detected temperature and the calculated battery temperature increase rate, with which the battery can be charged while preventing the battery temperature from excessively rising;
charging the battery with the retrieved allowable current value;
determining that charging has been completed when the battery temperature increase rate is relatively large and when a relatively low allowable current value has been selected at a relatively high frequency; and
concluding charging based on the determination that charging is completed.
According to the battery chargers and charging methods described above, charging current is controlled using a map of allowable current values with which a battery can be charged in order to prevent the battery temperature from excessively rising. The allowable current value is retrieved based upon the battery temperature and the battery temperature increase rate. The battery is charged with the allowable current value thus obtained and a nickel metal hydride battery, the temperature of which tends to excessively rise during charging, can be charged within a relatively short period of time without causing deterioration due to excessive temperature increases. Just before charging is completed, the temperature increase rate of the nickel metal hydride battery tends to substantially increase. Therefore, the battery is charged with a relatively low current when the charging operation is nearly complete. Consequently, it is possible to suppress xe2x80x9covershootxe2x80x9d after the completion of charging.
In another aspect of the present teachings, the completion of charging is determined, in particular, based on whether or not the battery temperature increase rate is relatively large and whether or not a relatively low current has been selected from the map at a relatively high frequency. That is, whether the battery is completely charged is determined based on whether or not the battery temperature increase rate is large and whether or not the increase rate is large, even though the charging current has been decreased. Due to this, it is possible to fully charge the battery without fear of overcharging and without being influenced by the remaining battery capacity, the battery temperature or other battery conditions.